INTERNAL-ROTATION IN CONJUGATED MOLECULES - SUBSTITUTED ETHYLENES AND BENZENES

Ab initio molecular orbital theory is employed to calculate the barrier to internal rotation in six conjugated ethylenes and the six analogous substituted benzenes. To correctly describe the loss of partial double bond character upon rotation, the inclusion of electron correlation is shown to be necessary. Such calculations are performed on all molecules with second-order Moller-Plesset perturbation theory (MP2) and the 6-311G** basis set, at geometries optimized by Hartree-Fock calculations at the 6-31G* level. The effects of further improvements in the theoretical treatment are assessed by calculations on the six substituted ethylenes and are found to change the barriers by up to 1 kcal mol-1. Agreement of better than 1 kcal mol-1 is also usually obtained between the calculations and experiment. In three cases where this level of agreement is not obtained, the experimental results are discussed in detail.